Piezoelectrical bending converter

ABSTRACT

Piezoelectrical bending converter with at least one monolithic layered composite, includes a piezoelectric-active ceramic layer with a lateral dimension change which may be generated by application of an electrical field and at least one further piezoelectric-active ceramic layer with a further lateral dimension change, different from the lateral dimension change. An electrode layer is arranged between the ceramic layers, for generation of the electric fields. The layered composite preferably comprises several piezoelectric-active layers and electrode layers arranged between the above. The shift is thus obtained as a gradient of the dimension changes in the direction of the stack of the layered composite. The dimension changes for the ceramic layers and the gradient may be adjusted by the generation of the electrical fields. In order to achieve a shift it is necessary not to have a piezoelectric-inactive layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the 35 USC 371 national stage of international applicationPCT/DE02/02183 filed on 14 Jun. 2002, which designated the United Statesof America.

FIELD OF THE INVENTION

The invention relates to a piezoelectric flextensional transducer withat least one monolithic multilayer system, having a piezoelectricallyactive ceramic layer with an electric-field-inducible lateraldimensional change and having at least one other piezoelectricallyactive ceramic layer with another lateral dimensional change inducibleby another electric field and different from said lateral dimensionalchange. A flextensional transducer of this kind is known from XiaopingL. et al., J. Am. Ceram. Soc., 84 (5), 996-1003 (2001).

BACKGROUND OF THE INVENTION

The piezoelectrically active ceramic layers of the monolithic multilayersystem of the known flextensional transducer are made of different leadzirconate titanate (PZT). The ceramic layers exhibit different d₃₁coefficients. Under the effect of an electric field, this produces ineach case a dimensional change (contraction) in the relevant ceramiclayer along a surface (lateral) stretch of the ceramic layer. The d₃₁coefficient is a measure of the inducible dimensional change anddepends, for example, on the ceramic material, the layer thickness andthe polarization of the ceramic layer.

In the multilayer system, there is no step change in the d₃₁coefficients between the ceramic layers in the stacking direction of themultilayer system. Instead the d₃₁ coefficient varies continuously inthe stacking direction, i.e. there is a d₃₁ coefficient gradient in thestacking direction.

As a result of the different d₃₁ coefficients of the ceramic layers, ord₃₁ coefficient gradient, deformation (deflection) of the multilayersystem occurs when the ceramic layers are exposed to approximatelyidentical electric fields. A stroke is produced in the stackingdirection of the multilayer system.

The actual dimensional change in each ceramic layer and therefore anymeasure of the deflection or stroke of the multilayer system depends onthe strength of the electric field induced in the ceramic layer. Togenerate the electric fields, the monolithic multilayer system has twoelectrode layers between which the two piezoelectrically active ceramiclayers are disposed. The electric fields in the ceramic layers cannot begenerated independently of one another. It is therefore relativelydifficult to produce a specific deflection of the multilayer system.

Flextensional transducers of the “Bimorph”, “Cymbal”, “Moonie”,“Multilayer” and “Rainbow” type are known from A. Dogan et al., IEEETrans. Ultrason., Ferroelectr., Freq. Contr., Vol 44, No. 3, 597-605,1997. To increase an effective stroke, a plurality of flextensionaltransducers of one of the types can be arranged to form a stack, eachindividual flextensional transducer of the stack contributing with itsdeflection or stroke to the effective stroke of the stack. Theflextensional transducers of the types mentioned above have a multilayersystem consisting of at least one piezoelectrically active and at leastone piezoelectrically inactive layer. To produce the deflection, amarked voltage swing is generated at an interface between the layers.However, the marked voltage swing reduces the reliability of theflextensional transducer. If the flextensional transducer iscontinuously stressed, this can easily result in failure of theflextensional transducer.

SUMMARY OF THE INVENTION

The object of the present invention is to specify a reliablepiezoelectric flextensional transducer whose stroke can be relativelyeasily adjusted compared to the known prior art.

To achieve this object, a piezoelectric flextensional transducer with atleast one monolithic multilayer system is specified, having apiezoelectrically active ceramic layer with an electric-field-inducedlateral dimensional change and having at least one otherpiezoelectrically active ceramic layer with another lateral dimensionalchange which can be induced by another electric field and which isdifferent from said lateral dimensional change. The piezoelectricflextensional transducer is characterized in that there is disposedbetween the ceramic layers at least one electrode layer for generatingthe electric fields.

The monolithic multilayer system consists, for example, of a pluralityof piezoelectrically active ceramic layers and internal electrode layersdisposed therebetween. Each of the inner electrode layers is used togenerate the electric fields in the adjacent piezoelectrically activeceramic layers. Each of the piezoelectrically active ceramic layers ispolarized in the thickness direction of the ceramic layer and thereforein the stacking direction of the monolithic multilayer system. Duringpolarization, opposite polarization directions are produced in adjacentceramic layers by applying alternating polarities to adjacent electrodelayers. Electrically driving the electrode layers during operation ofthe flextensional transducer, i.e. generating electric fields parallelto the polarization directions, produces lateral dimensional changesalong the ceramic layers. The ceramic layers are shortenedperpendicularly to the polarization directions and therefore to thethickness directions of the ceramic layers.

It is also conceivable for electrode layers electrically isolated fromone another to be disposed between adjacent piezoelectrically activeceramic layers, enabling identically oriented polarization directions tobe induced in the adjacent ceramic layers.

Monolithic means that the multilayer system is produced by co-firing ofthe ceramic layers and the electrode layers disposed therebetween. Forexample, to produce the monolithic multilayer system, a plurality ofgreen ceramic foils printed with electrode material are stacked one ontop of another, laminated, if necessary debindered and then co-fired.The green foils are selected e.g. such that co-firing results in ceramiclayer thicknesses of between 20 and 500 μm.

The underlying concept of the invention is that only piezoelectricallyactive ceramic layers are used to produce a deflection or stroke of theflextensional transducers. The stroke is obtained without apiezoelectrically inactive layer. To achieve this, the lateraldimensional changes that can be produced in the ceramic layers aredifferent. In order make the dimensional changes as adjustable aspossible, an electrode layer is disposed between every twopiezoelectrically active ceramic layers. The electrode layer is verythin compared to the ceramic layers and does not therefore act aspiezoelectrically inactive layers in the abovementioned sense. Using theelectrode layer, a defined polarization of the adjacent ceramic layersis produced in each case. The electrode layer is additionally used togenerate the electric fields required for the dimensional changes. Boththe polarizations and the electric fields or the strengths of theelectric fields can be more easily and precisely adjusted compared tothe known prior art. This is because, for example, any likelihood of amixed phase or void between adjacent ceramic layers being producedduring co-firing is greatly reduced by the electrode layer. Mixed phasesor voids of this kind have an unpredictable effect on the polarizationof a ceramic layer and likewise the electric field to which the ceramiclayer is exposed during operation.

In a particular embodiment, a gradient in the lateral dimensionalchanges of the ceramic layers can be produced in one stacking directionof the multilayer system. This means that during operation of theflextensional transducer, the magnitude of the dimensional changesvaries from one ceramic layer to the next in one direction in thestacking direction of the multilayer system. For example, the multilayersystem can consist of three piezoelectrically active ceramic layers.During operation, the size of the contraction of the three ceramiclayers reduces from ceramic layer to ceramic layer. Although thedimensional changes vary abruptly from ceramic layer to ceramic layer,the dimensional changes are preferably adjusted such that any voltageswings occurring between adjacent ceramic layers are much smaller thanin a system comprising a piezoelectrically active and apiezoelectrically inactive layer.

There are various ways of achieving different lateral dimensionalchanges. For example, the ceramic layers could have different d₃₁coefficients and the electric fields acting on the ceramic layers toachieve the deflection of the multilayer system during operation couldbe identical. It is also conceivable for the d₃₁ coefficients to be thesame, but the electric fields to be different.

The d₃₁ coefficient depends on the ceramic material of the ceramiclayers. In a particular embodiment, the ceramic layers are of the sameceramic material. This has the advantage, for example, that (if thepolarization of ceramic layers is sufficiently identical) virtually nobending as the result of a different coefficient of thermal expansion ofdifferent ceramic material occurs. There is no thermally induceddeflection. However, it is also conceivable for the ceramic layers to bemade of different ceramic material, this being advantageous if thecoefficients of thermal expansion of the ceramic materials are similar.

In a further embodiment, the ceramic layers have essentially identicallayer thicknesses. “Essentially identical” means that a tolerance of upto 10% is permissible. If the layer thicknesses of the ceramic layersare the same and the ceramic layers consist of identical ceramicmaterial, a different dimensional change can be produced by differentelectric field strengths acting on the ceramic layers, the polarizationbeing the same. Different dimensional changes are also accessible due tothe fact that the ceramic layers have different polarizations and areexposed to identical or similar electric field strengths, said fieldstrengths being advantageously selected such that there is no change inthe polarization of the ceramic layers (re-poling) during operation ofthe flextensional transducer.

In a particular embodiment, the ceramic layers have different layerthicknesses. Both for polarization of the ceramic layers and duringoperation of the flextensional transducer, the electrode layers can havethe same electric potentials applied to them with alternating wiring. Asa result, with identical electrical driving of the electrode layers,different dimensional changes can be produced in the ceramic layers.

In a further embodiment, the multilayer system has at least one means ofreducing any intrinsic stiffness of the multilayer system. By reducingthe intrinsic stiffness, the stroke of the multilayer system can beincreased. The means of reducing the intrinsic stiffness is inparticular a hole. The hole is disposed in the multilayer system in sucha way that, in the case of said multilayer system with a hole, lessenergy is required for a specific stroke than for a correspondingmultilayer system without a hole. The hole is e.g. a bore in or throughthe multilayer system in the stacking direction.

The multilayer system can have any footprint. The footprint can berectangular, for example, resulting in a strip-shaped multilayer system.A footprint in the shape of a regular hexagon is also conceivable. In aparticular embodiment, the multilayer system has a circular footprint.The multilayer system constitutes a disk bender, each of the ceramiclayers preferably being a disk. During polarization of a disk-shapedceramic layer in the thickness direction, the d₃₁ coefficient actsradially and in the circumferential direction of the ceramic layer. Thecircumference of the ceramic layer is reduced parallel to thepolarization direction due to the effect of an electric field. The d₃₁coefficient is advantageously superposed by the d₃₃ coefficient,resulting in an increase in the layer thickness. A displacement in theaxial direction occurs, thereby increasing the resulting stroke of theceramic layer.

Another feature of the disk bender is its good stability. The diskbender is self-supporting. A disk bender support is, for example, not apoint but extends across the circumference of the disk bender. A furtheradvantage is that the disk bender only has to be fastened. Unlike aflextensional transducer in strip format, for example, it does not needto be clamped. If the flextensional transducer is very rigidly clamped,this may result, for example, in mechanical damage to the flextensionaltransducer in the clamping region. No such problem arises with the diskbender. Further advantages compared to other types include increasedblocking force, increased rigidity and a relatively high resonantfrequency. In addition, tried and tested methods for manufacturingceramic multilayer actuators can be used for manufacturing the diskbender.

In a particular embodiment, a plurality of monolithic multilayer systemsare arranged in a stack. Specifically a plurality of multilayer systemsin the form of disk benders are arranged in a stack. The disk bendersare reciprocally disposed so that a considerable effective stroke can beachieved as the result of an additive superposition of the strokes ofthe individual disk benders, said disk benders being externallyfastenable via a casing. If the disk benders have a hole, they can alsobe fastened via a spindle passing through the holes.

To summarize, the invention provides the following main advantages:

-   -   No piezoelectrically inactive layer is required to achieve the        stroke.    -   In the multilayer system, a dimensional change gradient can be        produced, causing the stroke.    -   The sudden voltage changes occurring between the ceramic layers        are very small, thereby increasing the stability of the        flextensional transducer.    -   The electric fields to which the ceramic layers are exposed        during operation can be freely selected by the (separately        drivable) electrode layers between the piezoelectrically active        ceramic layers of the multilayer system.    -   The electrode layers prevent the formation of mixed phases and        voids at the interfaces of adjacent ceramic layers. The electric        fields and therefore the dimensional changes can be easily        adjusted.    -   In the multilayer structure, relatively low voltages can be        applied to the electrode layers to achieve the stroke.    -   A flextensional transducer with a multilayer system in the form        of a disk bender is inexpensive to manufacture, self-supporting        and stable. Because of the interaction of the d₃₁ and d₃₃        coefficients, it is characterized by a particularly large        stroke.    -   A high effective stroke can be achieved by stacking a plurality        of multilayer systems, in particular disk benders, one on top of        the other.

BRIEF DESCRIPTION OF THE DRAWINGS

The piezoelectric flextensional transducer with monolithic multilayersystem will now be presented with reference to a number of exemplaryembodiments and the associated figures. The figures are schematic andare not drawn to scale.

FIG. 1 shows a multilayer system in a lateral cross-section.

FIGS. 2 a to 2 c show the footprints of various multilayer systems.

FIGS. 3 a and 3 b show stacks of electrically driven multilayer systems.

DETAILED DESCRIPTION OF THE INVENTION

The piezoelectric flextensional transducer 1 has a monolithic multilayersystem 2 (FIG. 1). The multilayer system 2 consists of fourpiezoelectrically active ceramic layers 31 to 34. The ceramic layers areof the same ceramic material 315 to 345 in each case. The ceramicmaterial is a lead zirconate titanate (PZT). The polarizations 313 to343 of the ceramic layers 31 to 34 are approximately identical in termsof absolute value. The polarizations of adjacent ceramic layers areparallel to one another, but oppositely oriented. The d₃₁ coefficientsof the ceramic layers are identical and are approximately −350 pm/V. Thelayer thicknesses of the ceramic layers 31 to 34 are different,measuring between 50 and 150 μm and reducing in the stacking direction22 of the multilayer system 2. Inner electrode layers 41 to 43 aredisposed between the ceramic layers 31 to 34. The termination in thestacking direction 22 of the monolithic multilayer system is formed bytwo outer electrode layers 44 and 45. The overall thickness of themultilayer system 2 is 300 μm. The multilayer system 2 is a disk bender21 with a circular footprint 63 having a radius of approximately 5 mm(FIG. 2 c). To reduce the intrinsic stiffness, the disk bender 2 has ahole 23 with a radius of approximately 1.5 mm (FIG. 2 c). The hole 2 isa bore through the disk bender 2 in the stacking direction 22.

During operation of the flextensional transducer 1, potentials 411 to451 are alternately applied to the inner and outer electrode layers 41to 45. The potentials 411 and 431 are 100 V and the potentials 441, 421and 451 are 0 V. By means of the combination of different layerthicknesses of the piezoelectrically active ceramic layers and identicalpotential differences between the electrode layers, the ceramic layersare subjected to electric fields 312 to 342 of different strengths. Thisresults in different dimensional changes 311 to 314, a gradient 5 in thelateral dimensional changes being produced in the multilayer system 2 orin the disk bender.

A feature of another embodiment is that the layer thicknesses of theceramic layers 31 to 34 are essentially identical. To achieve differentlateral dimensional changes in the ceramic layers, different potentialdifferences and thereby electric fields of different strengths can beproduced between the electrode layers. The electrode layers areindividually driven.

Further embodiments differ from those described in having differentmultilayer system footprints. A rectangular footprint 61 (FIG. 2 a)results in a strip-shaped multilayer system. A hexagonal footprint 62 isalso conceivable, as indicated in FIG. 2 b.

In order to maximize the stroke, a plurality of multilayer systems 2 inthe form of disk benders 21 are arranged in a stack 7. The disk benders21 are reciprocally stacked. This means that electrically driving thedisk benders 21 results in an additive superposition of the strokesproduced in the individual disk benders to produce an overall effectivestroke. In one embodiment, a spindle 72 passing through the bores 23through the stacked disk benders 21 is used for fastening (FIG. 3 a).Alternatively, the stack 7 is disposed in an appropriately dimensionedcasing 71 (FIG. 3 b).

1. Piezoelectric flextensional transducer, comprising: at least onemonolithic multilayer system, comprising a first piezoelectricallyactive ceramic layer with a first lateral dimensional change inducibleby a first electric field; a second piezoelectrically active ceramiclayer with a second lateral dimensional change inducible by a secondelectric field and different in magnitude from said first lateraldimensional change; at least a third piezoelectrically active ceramiclayer with another lateral dimensional change inducible by a thirdelectric field and different in magnitude from said second lateraldimensional change; at least a first electrode layer disposed betweenthe first and second ceramic layers for generating the first and secondelectric fields; and at least a second electrode layer disposed betweenthe second and third ceramic layers for generating the second and thirdelectric fields, wherein the first electrode layer is laminated on thefirst ceramic layer and the second ceramic layer is laminated on thefirst electrode layer, wherein the second electrode layer is laminatedon the second ceramic layer and the third ceramic layer is laminated onthe second electrode layer, wherein the multilayer system has a circularfootprint, wherein a thickness direction of the ceramic layers defines astacking direction of the monolithic system, wherein a gradient in thefirst, second, and third lateral dimensional changes of the ceramiclayers, can be produced in the stacking direction of the multilayersystem, and wherein the multilayer system is self-supporting across acircumference of the circular footprint.
 2. Flextensional transduceraccording to claim 1, wherein each of the ceramic layers are of the sameceramic material and have an upper surface printed with electrodematerial, the electrode material of each ceramic layer constituting oneelectrode layer.
 3. Flextensional transducer according to claim 1,wherein the ceramic layers are of different ceramic material. 4.Flextensional transducer according to claim 1, wherein the ceramiclayers have essentially identical layer thicknesses and differentlateral dimensional changes are achieved in each of the ceramic layersby the first and second electric fields being of different strengths,and the multilayer system is free on any piezoelectrically inactivelayers between the piezoelectrically active first and second layers. 5.Flextensional transducer according to claim 1, wherein the ceramiclayers have different layer thicknesses, and the multilayer system isfree on any piezoelectrically inactive layers between thepiezoelectrically active first and second layers.
 6. Flextensionaltransducer according to claim 1, wherein the multilayer system has atleast one means for reducing any intrinsic stiffness of the multilayersystem.
 7. Flextensional transducer according to claim 6, wherein themeans for reducing the intrinsic stiffness is a hole.
 8. Flextensionaltransducer according to claim 1, comprising a plurality of monolithicmultilayer systems arranged in a stack.
 9. Flextensional transduceraccording to claim 1, wherein the ceramic layers are of the same ceramicmaterial, and the multilayer system is free on any piezoelectricallyinactive layers between the piezoelectrically active first and secondlayers.
 10. Flextensional transducer according to claim 1, wherein theceramic layers are of different ceramic material.
 11. Flextensionaltransducer according to claim 1, wherein the ceramic layers haveessentially identical layer thicknesses, and the multilayer system isfree on any piezoelectrically inactive layers between thepiezoelectrically active first and second layers.
 12. Flextensionaltransducer according to claim 1, wherein the ceramic layers havedifferent layer thicknesses.
 13. Flextensional transducer according toclaim 1, wherein the multilayer system has at least one means forreducing any intrinsic stiffness of the multilayer system. 14.Flextensional transducer according to claim 1, wherein the means forreducing the intrinsic stiffness is a hole.
 15. Flextensional transduceraccording to claim 1, comprising a plurality of monolithic multilayersystems arranged in a stack.
 16. Piezoelectric flextensional transducerwith at least one monolithic multilayer system, comprising: at leastthree adjacent piezoelectrically active ceramic layers defining themonolithic multilayer system; and at least one electrode layer locatedbetween each pair of the adjacent ceramic layers, each electrode layerfor generating electric fields for the pair of adjacent ceramic layers,wherein, a thickness direction of the ceramic layers defines a stackingdirection of the monolithic system, the electrode layers provide anelectric field that induces a lateral dimensional change in each of theceramic layers, each electric-field-induced lateral dimensional changebeing different in magnitude from the electric-field-induced lateraldimensional changes of each adjacent active ceramic layer and a gradientin the lateral dimensional changes of the three adjacent ceramic layersis produced in the stacking direction, each ceramic layer is polarizedin a thickness direction of the ceramic layer and therefore in thestacking direction of the monolithic multilayer system, duringpolarization of the ceramic layers, opposite polarization directions areproduced in adjacent ceramic layers by applying alternating polaritiesto adjacent electrode layers, electrically driving the electrode layersgenerates the electric fields parallel to the polarization directionsand produces the lateral dimensional changes along the ceramic layerswith the ceramic layers shortened perpendicularly to the polarizationdirections and therefore to the thickness directions of the ceramiclayers, during operation, a size of a contraction of each of theadjacent three ceramic layers reduces respectively from a first ceramiclayer to each successive one of the adjacent ceramic layers, themultilayer system has one of a circular, a hexagonal and a rectangularfootprint with each of the ceramic layers being of a corresponding oneof the circular, the hexagonal and the rectangular shape, and themultilayer system is free on any piezoelectrically inactive layersbetween the piezoelectrically active layers, and the assembly isself-supporting across the footprint.
 17. Flextensional transduceraccording to claim 16, wherein the assembly has the hexagonal footprintwith each of the ceramic layers being of the hexagonal shape and theassembly is self-supporting across a circumference of the footprint. 18.Flextensional transducer according to claim 16, wherein the assembly hasthe circular footprint with each of the ceramic layers being of thecircular shape and the assembly is self-supporting across acircumference of the footprint.
 19. Piezoelectric flextensionaltransducer, comprising: three piezoelectrically active ceramic layers ofthe same ceramic material, the three ceramic layers laminated one toanother, polarizations of the three ceramic layers being approximatelyidentical in terms of absolute value, the polarizations of three ceramiclayers being parallel to one another and oppositely oriented; innerelectrode layers laminated between adjacent ones of the three ceramiclayers; and outer electrode layers laminated on outside surfaces of theouter ones of the three ceramic layers, wherein, each of the threeceramic layers has a different lateral dimensional change inducible by arespective electric field generated by the electrode layers, and whereina gradient in the lateral dimensional changes of the ceramic layers isproduced in a stacking direction.